The invention relates to improvements in processes for converting mixtures of calcined clays into particles containing a crystalline zeolite molecular sieve component and a non-zeolitic component by reacting mixtures of calcined clays with sodium hydroxide solution. In particular, the invention relates to improvements in making zeolitic cracking catalysts useful in the fluidized catalytic cracking (FCC) of hydrocarbon feedstock from mixtures of calcined kaolin clays as described in U.S. Pat. Nos. 3,647,718 and 3,657,154.
Crystalline zeolitic molecular sieves are used in a wide variety of catalytic and adsorptive applications. Sieves of the faujasite type, especially ion-exchanged forms of zeolite Y, are well-known constituents of hydrocarbon conversion catalysts. In commercial practice, synthetic forms of zeolite Y are utilized as a component of such catalysts because naturally-occurring faujasite is highly limited in supply.
Synthetic zeolites of the Y-type are commerically available as finely divided high purity crystals. Present commercial use of such zeolites in the fluidized cracking of hydrocarbons requires that the zeolite crystals in the particles of catalyst be associated with a suitable matrix material such as a silica-alumina gel, clay or mixture thereof, to provide catalyst particles which operate at activity levels useful in present-day cracking units. The binder material also functions to impart attrition resistance to the catalyst particles. When preparing cracking catalysts from fine particle size crystals, the choice of a binder is limited by the fact that the binder must be thermally stable, provide access of gases or liquids to the zeolite crystals in the composite particles and result in particles of acceptable resistance to attrition.
The synthesis of a variety of zeolites from calcined clays, especially kaolin clay, is known. For example, metakaolin (kaolin clay calcined at a temperature of about 1200.degree. to 1500.degree. F.) will react with sodium hydroxide solution to produce sodium zeolite A. On the other hand, metakaolin can react with sodium silicate solutions under selected conditions to form synthetic zeolites of the faujasite type. When kaolin is calcined under more severe conditions, sufficient to undergo the characteristic exothermic reaction (for example calcination at about 1700.degree. to 2000.degree. F.), the calcined clay will react with sodium hydroxide solution under controlled conditions to synthesize faujasite-type zeolites.
The reaction between kaolin calcined to undergo the exotherm and sodium hydroxide in an aqueous reaction medium is quite sensitive to the history of the clay prior and during calcination. It is known that the addition of a minor amount of metakaolin relative to kaolin calcined to undergo the exotherm frequently assures that a desired amount of synthetic faujasite, especially zeolite Y having a desirably high SiO.sub.2 /Al.sub.2 O.sub.3 ratio, will be crystallized under commercially viable production conditions.
Processes for producing zeolitic cracking catalysts useful in moving bed and fluidized (FCC) cracking units utilize the concept of employing calcined clay reactant(s) in substantially the same size and shape as the desired catalyst product. Because the bodies are zeolitized directly without a separate binding step to composite the zeolite and binder component, various embodiments of such processing have become known as "in situ" processes. Reference is made to the following commonly assigned patents of Haden et al: U.S. Pat. Nos. 3,391,994; 3,433,587; 3,503,900; 3,506,594; 3,647,718; 3,657,154; 3,663,165 and 3,932,268.
In producing FCC catalysts by the in situ process, preformed spray dried microspheres consisting of kaolin clay calcined to undergo the exotherm are mixed with particles of metakaolin and a solution of sodium hydroxide to form a slurry, which is then aged, typically for 4-8 hours at 100.degree. F. and subsequently heated to crystallize a zeolite of the Y-type, typically by heating the aged slurry at about 180.degree. F. for 20 to 25 hours. Preparation of an FCC cacalyst in this manner and using metakaolin in the form of a powder and kaolin calcined to undergo the exotherm in the form of microspheres is described in U.S. Pat. No. 3,657,154. Similar use of metakaolin in the form of microspheres and kaolin calcined to undergo the exotherm in microspheres separate from the microspheres composed of metakaolin is described in U.S. Pat. No. 3,647,718.
In practice of the procedure of U.S. Pat. No. 3,647,718, the microspheres composed of metakaolin and microspheres composed of kaolin calcined to undergo the exotherm are mixed before aging in a reactor vessel as a batch reaction. Subsequently the reactor charge is heated to effect the crystallization. The crystallization is also on a batch scale. All of the calcined clay reactants (e.g., microspheres of kaolin calcined to metakaolin condition and microspheres of kaolin calcined to undergo the exotherm) are mixed prior to aging. Crystallization time generally substantially exceeds aging time. Criteria for selecting ratios of microspheres of metakaolin to microspheres of clay calcined to undergo exotherm are set forth in U.S. Pat. No. 3,647,718 at col. 6, lines 3 to 18. Silica originally in the microspheres is leached or extracted during the reaction, producing a sodium silicate mother liquor which is removed in whole or in part from the crystallized microspheres. The microspheres, now containing a mixture of sodium zeolite Y and a silica-depleted (alumina enriched) residue of calcined clay, are subsequently subjected to ion-exchange treatment, typically with ammonium ions or ammonium and rare earth ions, to replace sodium ions with cations used in the exchange treatment(s).